Overspeed braking devices for shafts are relatively well known. These devices usually include a pawl which rotates with the shaft and can travel outwardly as a result of centrifugal force to engage a stationary lug at a predetermined overspeed and thereby brake the shaft's rotation. Overspeed safety brakes of this type are generally used for stopping shafts that rotate at relatively low speeds on the order of about 200 rpm and find application, for example, in lifts, hoists, loading bridges for aircraft, elevators, and turrets. Obviously, however, there are other applications which would be obvious to those of ordinary skill in the art and the foregoing list is only exemplary. Some prior art overspeed braking devices are disclosed in U.S. Pat. Nos. 4,015,696 to Lichti (one of the present inventors); 3,934,682 to Hedstrom; 3,768,609 and 3,695,399 to Laing; 3,729,071 to Laing et al; 3,576,242 to Mumma; and 3,415,343 to Svensson.
In most of the foregoing brake mechanisms, a rotating pawl is continuously pivoted outwardly by centrifugal force against an opposing resilient force from a spring until the pawl is moved far enough radially outwardly such that it engages a stationary stop. Thus, the shape of the pawl, its weight, and the attaching spring are very critical in order for the device to actuate at the proper speed. In all of the devices in which the shaft is stopped upon engagement of the pawl with a stationary stop, the stopping torque is transmitted through the pawl and to the shaft which the pawl is journalled and rotates and from the shaft to some type of braking mechanism. This arrangement provides a large, undesirable shearing force on the pawl shaft. In addition, this common pawl system does not function well at very low speeds (under 200 rpm) because the entire pawl is not free to travel outwardly, but instead is constrained to pivot about an arc. Other disadvantages of many of the prior art overspeed braking devices used to stop large shafts include their relatively large size, complexity, and inaccuracy. Furthermore, many of these devices require disassembly of the system for resetting once the brake has been engaged.